1. Field of the Invention
The present invention relates to a method, an electrode and a device according to the preamble to appended claims 1, 14 and 17.
2. Description of the Related Art
In wet etching methods, a fluid is usually employed, which comprises an etchant capable of reacting in an etching manner with an etching material. In chemical etching, the etchant reacts spontaneously with the etching material, and in electrolytic etching, the etchant reacts by electrochemical reactions on the surface of the etching material when an etchant, with voltage applied, carries a current.
When the purpose of etching is to provide a structure in the etching material by etching away selected parts thereof, the etching material surface which is not to be etched away is usually coated with an etching preventing layer, a so-called mask or resist, which is insensitive or less sensitive to the etchant than the etching material. Such an etching preventing layer is produced in a multistage operation with an initial coating step and a subsequent step for partially removing the layer.
A common etching preventing layer is a photoresist. When using a photoresist, the surface of the etching material is coated in a first step with the photoresist which is sensitive to light. In a next step, the surface areas of the photoresist where etching is to be carried out are exposed to light, and in a subsequent step, these areas are developed and dissolved, thus uncovering the areas of the etching material that are to be etched. This method is currently used, for instance, when manufacturing printed circuit boards for etching away material to form gaps between conductors. The manufacture of an etching preventing layer in prior-art manner thus is complicated and time-consuming.
In wet etching methods, mainly in chemical etching, so-called underetching takes place owing to the isotropic etching properties, i.e. etching away material under the surface coated with an etching preventing layer. In consequence, it is not possible to make gaps having a greater depth than width by pure chemical etching. Nor is it possible in electrolytic etching to etch, in small dimensions, gaps the depth of which exceeds the width. The possibilities of making narrow gaps, for instance, to arrange conductors close to each other, are thus limited when applying wet etching methods. It is also not possible today to achieve even structures by wet etching methods, for instance, gaps with straight walls whose width or depth is below 1 xcexcm.
An object of the present invention is to provide improvements in connection with wet etching, especially when etching in very small dimensions, below 1 mm.
A special object is to provide an improved method of etching selected parts of a surface.
One more object is to permit quicker manufacture of etched patterns in a surface.
According to the invention, these and other objects that will appear from the following specification are now achieved by a method, an electrode and a device which are of the types stated by way of introduction and which, in addition, have the features stated in the characterising clause of claims 1, 14 and 17, respectively.
Thus, the invention is based on a new type of electrode for etching selected portions of an etching surface. The electrode has electrically conductive electrode portions in selected portions of an electrode surface. These electrode portions constitute an electrode pattern. When etching according to the invention using such an electrode which is directed towards the etching surface, depressions are made, which form an etching pattern corresponding to the electrode pattern. The etching pattern seen from the electrode will be inverted in relation to the electrode pattern seen from the etching surface.
According to the invention, an etching method for etching selected parts of a surface has thus been provided, without the surface needing to be covered with an etching preventing layer. Such an electrode can be used several times to etch in this manner a plurality of products successively. This enables great improvements in serial etching, both in respect of production times and production costs.
According to a preferred embodiment of the invention, the electrode portions let electromagnetic radiation through. By the electrode portions being permeable to electromagnetic radiation, it is possible to irradiate the etching material through the electrode during etching. The etching material is coated with a passivating layer, which reduces or prevents the capability of the etchant of etching the etching material. The passivating layer is such as to be dissolved in a chemical reaction when exposed to radiation.
According to an alternative embodiment, the passivating layer is exposed to radiation from below through the etching material. The wavelength of the radiation is adapted to the etching material so that the radiation at least partly penetrates the etching material.
It is preferred for the average intensity of the radiation to be so low that the temperature of the passivating layer is kept essentially constant.
According to a preferred embodiment of the present invention, a substance forming a passivating layer on the etching material is added to the etchant. The substance is selected so that the passivating layer is dissolved when exposed to electromagnetic radiation. When the radiation hits the passivating layer, it will be ionised and dissolved.
By providing, according to a preferred embodiment, the substance dissolved in the etchant, the passivating layer will continuously form on the surfaces of the etching material that are not exposed to radiation. As a result, a high anisotropy of the etching is achieved.
According to a preferred embodiment of the invention, the electromagnetic radiation is in the wavelength range 0.01-50 micrometer and preferably in the wavelength range 0.1-10 micrometer. There are a number of light sources that may be used.
In a preferred embodiment, the electrode has electrically insulating portions between the electrode portions. This results in more accurate control of the etching process since distinctly defined electric fields are formed. This is particularly advantageous when etching in small dimensions, below 100 xcexcm, and yields particularly good conditions for achieving a directed etching effect (anisotropic etching).
Between the electrode portions, the electrode preferably has portions which do not let radiation through. These portions are preferably also electrically insulating portions. Radiation falls only on the portions of the etching surface that are exposed to an electric field. This results in improved anisotropic etching.
The passivating substance can be selected from, for instance, the group consisting of iodine, halide salts, thiosulphates, thiocyanates, ammonia and amines.
In an advantageous embodiment, the insulating portions of the electrode comprise an insulating layer which is applied to the etching surface of the electrode between the electrode portions, which are defined by the insulating layer. This makes it possible to design the electrode in an extremely simple fashion, for instance by an insulating layer being applied directly to a conductive electrode surface. For instance, the layer can be produced in the same manner as a photoresist layer.
According to a special aspect of the invention, use is made of a chemically etching fluid, in which the etchant, which constitutes the active substance, is present in a diluted solution. In this case, extremely good results can be produced, such as exact etchings in small dimensions. This aspect of the invention is based on the surprising discovery that an etching fluid, which has been diluted to have a negligible etching effect, can be used for anisotropic etching under the action of an electric field.
In this connection, etching of an electrically conductive etching material is carried out by means of an etchant, which is present in a solution which is diluted to such an extent that it cannot be practically used for chemical etching. The etchant concentration is so low that there occur only occasionally such reactions between the etchant and the etching material as result in the removal of atoms from the etching material. By an electric field being provided in the etchant solution between the electrode and a surface portion of the etching material, a local concentration of etchant to the surface portion of the etching material forms. This results in a pronounced increase of the etching rate of the etchant while at the same time the etching direction of the etchant is affected.
The invention concerns etching of an electrically conductive material, the etching material. Tests have been made with different metals, such as Cu, Ni, Ti, Al and Cr, but the inventive method is expected to function for other conductive materials, such as alloys, and on semiconductors. The electric conductivity of the etching material should be such that an electric field can be generated in the diluted solution between the etching material and an electrode.
The crystal structure of the etching material is not critical, and the etching material can thus be monocrystalline as well as polycrystalline.
The etchant should be capable of reacting, when dissolved, in an etching way with an etching surface, intended for etching, of the etching material. Besides, it is expected that the etchant should be of such a character as to be affected kinetically by an electric field to enable a local concentration of the etchant.
An important feature according to the special aspect is that the etchant is present in a solution at a low concentration. In view of experiments that have been carried out, the desired anisotropic etching effect seems difficult to achieve with etchant concentrations above 200 mM. However, a lower concentration limit for good function could not be determined. Besides, it is expected that the etchant must have sufficient movability in the solution to permit a local concentration of the etchant.
The electric field is assumed to have two functions, on the one hand, to concentrate the etchant locally and, on the other hand, to accelerate etching, of which the first-mentioned function is currently assumed to be the most important one.
It is assumed that the electric field should be directed towards the surface of the etching material that is to be etched. To make it possible to locally raise the concentration of the etchant, the extent of the electric field at the surface that is to be etched should be relatively limited, which in a favourable way is achieved with the inventive electrode.
The invention is especially, but not exclusively, directed to the manufacture of small structures, smaller than 1 mm, and is, together with a chemically etching etchant in a low concentration, particularly advantageous when manufacturing structures, such as depressions, in the order of 100 xcexcm and smaller, especially smaller than 50 xcexcm in respect of etching width as well as etching depth.
By the solution having an extremely low concentration of etchant and a relevant etching process being carried out while being affected by an electric field, the etching process achieves essentially improved controllability and anisotropy compared with prior-art wet methods.
An important property is that, according to the special aspect, it is possible to etch grooves and gaps having a larger depth than width. The ratio of depth to width of an etched gap has in experiments been measured to be 3.5:1 when etching in a thin copper film.
The method according to the special aspect further does not have a critical sensitivity to concentration variations within an efficient concentration range. Experiments have shown that around a concentration value which gives good etching results for a certain combination of etchant-etching fluid, it is possible to change the concentration value by a factor two without significantly deteriorating the etching result.
The etchant, in a diluted solution, is preferably in connection with etching in such a state that its capacity of etching spontaneously, i.e. in the absence of electric field, is restricted to an etching rate of 5 nm/s. For good anisotropy it is advantageous to restrict the spontaneous etching capacity of the etchant to at most 4 nm/s and more preferably to 3 nm/s and below.
The preferred concentration of the etchant is at most 200 mM, preferably at most 100 mM, more preferably at most 20 mM and most advantageously at most 10 mM. It may be generally said that the controllability of the etching process, especially when etching small structures, increases with a reduced etchant concentration. In some contexts, it has been found advantageous to have etchant concentrations below 2 mM, and particularly advantageous to have etchant concentrations of 1 mM and below.
The etchant can preferably be defined as an ionic substance capable of reacting in an etching manner with the etching material. The concentrations stated in connection with the invention concern the concentration of the etchant which is active according to the invention.
It is preferred for the voltage between the electrode and the etching material to be at least 0.5 V, preferably at least 1 V and more preferably at least 1.5 V, and at most 10 V, preferably at most 5 V and more preferably at most 3 V. Good results have been achieved in the range 2 V-2.8 V, and extremely good results have been achieved in the range 2.4-2.6 V. It is difficult to determine a lower limit for the voltage required for etching, and the above-mentioned values are values where a practically usable etching rate is achieved. However, it is important for the voltage not to be equal to or opposed to the electrochemical potential between the etching material and the electrode, which causes all etching action to cease.
The strength of the electric field is advantageously such that the etchant, in the diluted solution, is given an increased etching rate, which preferably is doubled, and more preferably is at least ten times higher than in the absence of the electric field.
In a preferred embodiment, etching occurs during a plurality of first periods, between which the electric field changes, in which case the electric field, between said first periods, is given an inversed direction during second periods. Consequently, residual products are released in an etched depression during the second periods, which again enables a directed etching effect during the next first period.
For the provision of high anisotropy and an exact etching result, it is particularly advantageous if the electrode portions are arranged very close to the etching material. The distance is preferably smaller than 2000 xcexcm, and more preferably smaller than 1000 xcexcm. To permit good transport of etching fluid, i.e. etchant in solution, it is preferred that the distance to the electrode, including any insulating layer, is greater than 1 xcexcm, preferably greater than 5 xcexcm.
As mentioned above, the invention has particular advantages when using a spontaneously etching etchant. However, there is nothing to prevent the inventive idea from being used also in electrolytic etching.